Nuclear organization of orexinergic neurons in the hypothalamus of a lar gibbon and a chimpanzee

Abstract

Employing orexin-A immunohistochemical staining we describe the nuclear parcellation of orexinergic neurons in the hypothalami of a lar gibbon and a chimpanzee. The clustering of orexinergic neurons within the hypothalamus and the terminal networks follow the patterns generally observed in other mammals, including laboratory rodents, strepsirrhine primates and humans.The orexinergic neurons were found within three distinct clusters in the ape hypothalamus, which include the main cluster, zona incerta cluster and optic tract cluster. In addition, the orexinergic neurons of the optic tract cluster appear to extend to a more rostral and medial location than observed in other species, being observed in the tuberal region in the anterior ventromedial aspect of the hypothalamus. While orexinergic terminal networks were observed throughout the brain, high density terminal networks were observed within the hypothalamus, medial and intralaminar nuclei of the dorsal thalamus, and within the serotonergic and noradrenergic regions of the midbrain

Orexinergic neurons in the hypothalami of an Asiatic lion, an African lion, and a Southeast African cheetah

“…Despite this consistency, there are indications of variations in the organization of this system. The current study of the felids, along with that of the lar gibbon and chimpanzee (Williams et al, 2022), indicates that one variation is the presence of orexinergic neurons in the ventromedial supraoptic hypothalamus, which we term the supraoptic orexinergic cluster. The second variation of significance is the presence of the distinct parvocellular orexinergic cluster in the Cetartiodactyls, African elephant, and the three felids studied herein (Table 3).…”

“…Indeed, across most mammals that have been investigated, orexinergic neurons have not been reported in this region of the hypothalamus (Table 3 ). The one exception is the report of similar orexinergic neurons being observed in the ventromedial supraoptic region of the lar gibbon and chimpanzee (Williams et al., 2022 ). In the lar gibbon and chimpanzee, these ventromedial supraoptic orexinergic neurons were described as a medial extension of the optic tract cluster, as they were few in number (Williams et al., 2022 ).…”

“…Within the ventral half of the medial preoptic region (medial to the fornix), a low density of orexinergic neurons was observed in all three felids studied ( Figures 1c-f, 2d-i, 6, and 7), which we term the 6b,d,f, and 7). The passage of the fornix was used to distinguish between the supraoptic orexinergic cluster (medial to the fornix) and the optic tract cluster (lateral to the fornix in the lateral supraoptic region), as in most mammals no orexinergic neurons are observed in this region of the hypothalamus (although see Williams et al [2022], where a similar distribution of orexinergic neurons was observed in the ventral half of the medial preoptic region in the lar gibbon and chimpanzee).…”

“…In several species of Cetartiodactyls (Iqbal et al., 2001 ; Ettrup et al., 2010 ; Dell et al., 2012 ; Dell, Karlsson, et al., 2016 ; Dell, Patzke, et al., 2016 ; Davimes et al., 2017 ; Malungo, Gravett, Bhagwandin, Davimes, & Manger, 2020 ) as well as the African elephant (Maseko, Patzke, Fuxe, & Manger, 2013 ), a novel cluster of small orexinergic neurons, termed the parvocellular cluster (Dell et al., 2012 ), has been observed medial to the fornix. In the lar gibbon and chimpanzee, the orexinergic neurons of the optic tract cluster have been observed to extend into the ventromedial supraoptic region (Williams et al., 2022 ). Lastly, in some species of rodents, potential orexinergic neurons have been observed in the paraventricular and supraoptic nucleus (Nixon & Smale, 2007 ).…”

Cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations in the brain of the lesser hedgehog tenrec (Echinops telfairi)

“…It has been proposed that the orexinergic neurons play a regulatory role between arousal and satiety, and in species that need to stay awake to ingest a large volume of low‐calorie content food the parvocellular cluster may augment this function (Malungo et al, 2020). Other variations in the organization of the orexinergic system across mammals include the absence of the optic tract cluster in microchiropteran bats (Kruger et al, 2010), and the medial expansion of the optic tract cluster in the lar gibbon and chimpanzee (Williams et al, 2022). Despite these variations in the organization of the orexinergic system, across Afrotheria, and Eutherian mammals more broadly, there appears to relatively little evolutionary plasticity in the nuclear organization of this hypothalamic neural system.…”